Over-voltage protection circuit

0001This is a divisional of U.S. patent application Ser. No. 11/371,686 filed on Mar. 9, 2006, which is a continuation of U.S. patent application Ser. No. 10/261,038 filed on Sep. 30, 2002 (now U.S. Pat. No. 7,035,070), which claims the benefit of priority from U.S. Provisional Application Ser. No. 60/325,551 filed on Oct. 1, 2001. By this reference, the full disclosure, including the drawings, of U.S. Provisional Application Ser. No. 60/325,551 is incorporated herein.

FIELD OF THE INVENTION

0002The present invention relates to energy storage components used in battery-operated devices. In particular, the present invention relates to protection against over-voltage of such energy storage components within the operational limits of a battery-operated device while charging.

BACKGROUND OF THE INVENTION

0003Many mobile devices, such as cellular telephones, personal digital assistants (PDAs), and other handheld computing and communicating devices, rely upon standard energy storage devices, such as battery cells, for providing power on which to operate.

0004Though disposable battery cells, such as alkaline cells, are a well-known and reliable technology, it is common in such mobile devices to employ rechargeable battery cells. These rechargeable batteries depend on a number of known cell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cells are known to those of skill in the art, as are some of their deficiencies.

0005Although some mobile devices are able to function with standard off-the-shelf rechargeable batteries, many use a specialised rechargeable battery made particularly for that make and model of mobile device. A charging device is necessary in order to recharge the mobile device's battery. Such a charging device may be a dedicated device, or may be integrated into an existing accessory, such as a cradle. The life of the battery can be drastically curtailed by improperly charging, or over discharging the battery.

0006Over-voltage protection circuits are commonly used to prevent a voltage across an energy storage device, such as a battery, from exceeding a set predetermined, or threshold, voltage. Such an energy storage device can comprise a plurality of energy storage components. Presently, over-voltage protection is typically achieved by connecting resistors in parallel with the energy storage device. In such over-voltage circuits, current continuously flows through the resistors whether the terminal voltage is above or below the set predetermined voltage, resulting in significant wasted power. Such a conventional configuration is illustrated in <figref idref="DRAWINGS">FIG. 1</figref>.

0007The energy storage devices <b>102</b>, <b>104</b> illustrated in <figref idref="DRAWINGS">FIG. 1</figref> are super capacitors, showing an example of a particular energy storage device. However, those of skill in the art will appreciate, the energy storage devices can be any suitable device, such as Ni-Cad, Ni-MH, and Li-Ion cells, for example.

0008<figref idref="DRAWINGS">FIG. 1</figref> shows a typical over-voltage protection circuit that is well known in the art. In this circuit <b>100</b>, energy storage devices <b>102</b>, <b>104</b> are connected in series. Each energy storage device has a parasitic internal leakage current. The magnitude of the leakage current may vary over a range of values, even among energy storage devices from the same manufacturing batch. These varying leakage rates result in the voltage across different energy storage devices decreasing at different rates. When the energy storage devices <b>102</b>, <b>104</b> are charged, the energy storage device with the lower leakage rate, and hence the greater voltage, can exceed the maximum voltage specified for that energy storage device before the combined voltage of both energy storage devices reaches a desired terminal voltage. Resistors <b>106</b>, <b>108</b> are placed in parallel with energy storage devices <b>102</b>, <b>104</b> respectively in order to equalise the respective voltage drops across each energy storage device. Charging leads <b>110</b> are shown in the drawing, for connecting a charging circuit (not shown) to the energy storage devices.

0009As one skilled in the art can appreciate, the resistors act to increase the total current flowing through each energy storage device, since the resistors are effectively in parallel with the parasitic resistance of the energy storage devices. This causes the energy storage devices to discharge any excess charge faster than if the resistors were not present. The resistor values are normally chosen so that the current in each resistor is much greater than the largest specified internal leakage current of the individual energy storage device. Given that the resistors typically come from the same manufacturing batch and are quite closely matched in value (within a few percent), the rate at which the voltage of the energy storage devices decrease is therefore more closely matched than if the resistors were absent.

0010However, this configuration results in continually wasted power since current is constantly flowing through the resistors and the current in each resistor is greater than the leakage current of the capacitor. A more power-efficient solution is required.

0011It is therefore desirable to provide a configuration that allows current to flow only when an energy storage component is above a predetermined voltage and thereby conserve power.

SUMMARY OF THE INVENTION

0012It is an object of the present invention to obviate or mitigate at least one disadvantage of previous over-voltage protection circuits, particularly those provided for use with handheld or mobile devices.

0013In an aspect of the invention, a protection circuit to prevent over-voltage of an energy storage device while being charged is provided. The energy storage device can be, for example, a super capacitor, or a lithium-ion battery.

0014In a first aspect, the present invention provides an over-voltage protection circuit for connection to a charging circuit for maintaining a voltage across an energy storage device at or below a predetermined voltage during charging. The circuit comprises an over-voltage protection device and an over-voltage protection controller. The over-voltage protection controller connects the over-voltage protection device in parallel with the energy storage device only when the voltage across the energy storage device exceeds the predetermined voltage, so as to draw excess charge from the energy storage device.

0015The over-voltage protection controller can comprise a switch actuated in response to an over-voltage condition at the energy storage device. The controller can further comprise an over-voltage detector coupled to the energy storage device and to the switch, which causes the switch to be actuated when a voltage measured across the energy storage device exceeds the predetermined voltage. This over-voltage detector and switch can be integral with one another.

0016In general, the over-voltage protection device either dissipates excess charge drawn from the energy storage device, or temporarily stores the excess charge.

0017A resistor is an example of an over-voltage protection device that dissipates excess charge drawn from the energy storage device. A zener diode or a shunt resistor may also be used as a dissipating over-voltage protection device, with the added advantage that each of these components can also act as the over-voltage protection controller. If either of these two is used in the over-voltage protection circuit, the use of a resistor or other over-voltage protection device is optional since the zener diode and shunt resistor act as both over-voltage protection controller and over-voltage protection device. In the case of the zener diode, a switch is preferably connected to the zener diode; the switch is closed during charging and open otherwise.

0018In a particular embodiment, an over-voltage protection circuit comprises a shunt regulator (occasionally called a voltage reference) in series with a resistor, and these are in parallel with an energy storage device. The shunt regulator prevents the voltage of the energy storage device from rising above a set predetermined voltage and only allows current to flow through it and the resistor when the voltage of the energy storage device is at or above the predetermined voltage, thereby conserving power.

0019A capacitor and an inductor are both examples of an over-voltage device that temporarily stores excess charge drawn from the energy storage device. Since these devices do not generally dissipate charge, a dissipation controller is preferably used in such configurations to dissipate the excess charge stored in the over-voltage protection device. The dissipation controller can comprise a dissipation switch that connects the over-voltage protection device to ground in order to dissipate the stored charge. Preferably, the dissipation controller also comprises a dissipation control mechanism that actuates the dissipation switch so as to connect and disconnect the over-voltage protection device from the dissipation controller.

0020There are alternative embodiments of the present invention that can be used in situations where the energy storage device comprises a plurality of energy storage components. An over-voltage device that temporarily stores excess charge is advantageously used in such instances to avoid over-voltage by balancing charge between the plurality of energy storage components.

0021Consider the exemplary case of a capacitor being used as an over-voltage protection device for two energy storage components. The over-voltage protection controller then comprises first and second switches coupled to the first and second energy storage components, respectively, and connected in series to either end of the capacitor. The switches are actuated, during over-voltage, so as to connect or disconnect the capacitor to each energy storage component in order to balance charge between them. One or more over-voltage detectors may be used in order to detect when over-voltage occurs. This detector can also control the actuation of the switches. Alternatively, an actuating means can be provided that actuates the connection or disconnection of the capacitor to each energy storage component at a regular time interval.

0022Next, consider the exemplary case of an inductor being used as an over-voltage protection device for two energy storage components. The over-voltage protection controller comprises first and second switches coupled to the first and second energy storage components, respectively, and connected in series with the inductor with respect to the charging circuit. The over-voltage protection controller preferably further comprises first and second diodes connected in parallel with the first and second switches, respectively. The switches are actuated, during over-voltage, to connect or disconnect the inductor to each energy storage component in order to balance charge between them. One or more over-voltage detectors may be used in order to detect when over-voltage occurs. This detector can also control the actuation of the switches. Alternatively, an actuating means can be provided that actuates the connection or disconnection of the capacitor to each energy storage component at a regular time interval.

0023In another aspect of the invention, an over-voltage protection circuit is provided for connection to a charging circuit for use with a handheld device for maintaining a voltage across an energy storage device at or below a predetermined voltage so as to avoid over-voltage during charging. The circuit comprises an over-voltage protection device and an over-voltage protection controller. The over-voltage protection controller connects the over-voltage protection device in parallel with the energy storage device in response to an over-voltage condition at the energy storage device, so as to draw excess charge from the energy storage device. The over-voltage protection circuit is connected to charging leads, which are connected to the charging circuit.

0024Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

0025Embodiments of the present invention will now be described, by way of example only, with reference to the attached figures, wherein:

0026<figref idref="DRAWINGS">FIG. 1</figref> illustrates a typical over-voltage protection circuit;

0027<figref idref="DRAWINGS">FIG. 2</figref> illustrates a block diagram of an embodiment of the invention;

0028<figref idref="DRAWINGS">FIG. 3</figref> illustrates an embodiment of the invention;

0029<figref idref="DRAWINGS">FIG. 4</figref> illustrates an embodiment of the invention;

0030<figref idref="DRAWINGS">FIG. 5</figref> illustrates an alternative embodiment of the invention; and

0031<figref idref="DRAWINGS">FIG. 6</figref> illustrates a further alternative embodiment of the invention.

DETAILED DESCRIPTION

0032Generally, the present invention provides an over-voltage protection circuit for protection against over-voltage of an energy storage device while charging. The circuit advantageously operates within the operational limits of a battery-operated device, such as a mobile or handheld device.

0033An over-voltage protection circuit according to the present invention comprises an over-voltage protection device, and an over-voltage protection controller. The controller allows current to flow to the over-voltage protection device only when an energy storage device is experiencing over-voltage. In allowing current to flow to the over-voltage protection device only when the voltage across the energy storage device is above a predetermined voltage, power conservation is achieved.

0034<figref idref="DRAWINGS">FIG. 2</figref> illustrates an embodiment of the present invention in block diagram form. In particular, an over-voltage protection circuit <b>112</b> is illustrated, which is preferably connected in parallel to a charging circuit (not shown) via charging leads <b>110</b>. The over-voltage protection circuit <b>112</b> is used during battery charging for maintaining a voltage across an energy storage device, such as a battery, below a predetermined voltage. The over-voltage protection circuit <b>112</b> comprises an over-voltage protection controller <b>114</b> and an over-voltage protection device <b>116</b>.

0035The over-voltage protection controller <b>114</b> can comprise any device that is actuated in response to a voltage measured between nodes A and C that meets or exceeds a predetermined voltage, and connects the over-voltage protection device <b>116</b> in parallel with the energy storage device <b>102</b> when actuated. Consequently, current is conducted to the over-voltage protection device <b>116</b> and charge drawn from the energy storage device <b>102</b> only when over-voltage occurs. The over-voltage protection controller <b>114</b> can comprise a switch, which is actuated in response to a voltage measured between nodes A and C that meets or exceeds the predetermined voltage. When the switch is actuated in response to an over-voltage condition, current is conducted to the over-voltage protection device <b>116</b> and charge is drawn from the energy storage device <b>102</b>. The switch can be, for example, a field effect transistor (FET), relay switch, bipolar junction transistor (BJT) or multiplexer (MUX). The switch preferably intrinsically comprises an over-voltage detector that causes the switch to be actuated when a voltage measured between nodes A and C exceeds a predetermined voltage. Alternatively, a separate over-voltage detector can be used in conjunction with the switch.

0036The over-voltage protection device <b>116</b> draws charge from the energy storage device <b>102</b> experiencing over-voltage. The over-voltage protection device <b>116</b> can comprise any device that is able to accept the drawn excess charge and dispose of it. The over-voltage protection device <b>116</b> can dissipate the energy itself, for example if a resistor is used. Alternatively, the over-voltage protection device <b>116</b> may temporarily store the excess charge, for example if a capacitor or inductor is used, then transfer it elsewhere to be dissipated. In the latter case, the over-voltage protection device <b>116</b> may temporarily store such charge until it is connected to a dissipation controller (not shown), at which time the charge stored therein may be dissipated in an appropriate manner, as will be well known to one skilled in the art.

0037For example, the dissipation controller can comprise a circuit having a dissipation switch that connects said over-voltage protection device to ground in order to dissipate the stored voltage. Preferably, this dissipation controller will also comprise a dissipation control mechanism that actuates the dissipation switch so as to connect and disconnect the over-voltage protection device <b>116</b> from the dissipation controller according to appropriate conditions.

0038Of course, the energy storage device <b>102</b> may, in fact, comprise a plurality of energy storage components connected in series. In such a case, a separate over-voltage protection circuit <b>112</b> can be connected in parallel to the terminals of each energy storage component in order to achieve a similar result as described in the embodiments above.

0039<figref idref="DRAWINGS">FIG. 3</figref> illustrates a presently preferred embodiment of the present invention, showing an over-voltage protection circuit <b>112</b>. In the embodiment shown in <figref idref="DRAWINGS">FIG. 3</figref>, a shunt regulator <b>118</b> is used as the over-voltage protection controller <b>114</b>. The use of a shunt regulator is advantageous in that shunt regulators have very sharp ‘turn-on’ characteristics. Suppose, for example, that a shunt regulator is chosen whose rated threshold voltage is below, but preferably near, the maximum specified voltage of the energy storage device <b>102</b>. When the voltage measured across the energy storage device <b>102</b> is below the threshold voltage of shunt regulator <b>118</b>, negligible current will flow through the shunt regulator <b>118</b>. If the voltage of the energy storage device <b>102</b> rises during charging to the threshold voltage of the shunt regulator <b>118</b>, the shunt regulator causes current to flow through it and through resistor <b>120</b>. Any excess energy is dissipated primarily across the resistor <b>120</b>, which is employed in this example as the over-voltage protection device <b>116</b>. Current continues to flow through the shunt regulator <b>118</b> until the voltage of the energy storage device <b>102</b> falls below the threshold voltage of the shunt regulator. The shunt regulator <b>118</b>, therefore, acts as both a switch and an over-voltage detector in this embodiment. If the leakage resistance in the shunt regulator <b>118</b> is suitable, the shunt regulator <b>118</b> can also perform the function of the over-voltage protection device <b>116</b>, thereby obviating the need for resistor <b>120</b>.

0040Although <figref idref="DRAWINGS">FIG. 3</figref> illustrates an exemplary embodiment of the invention, many alternative embodiments are possible. The energy storage device <b>102</b> may comprise a plurality of energy storage components connected in series and a separate over-voltage protection circuit <b>112</b> can be connected in parallel to the terminals of each energy storage component in order to achieve a similar result as described in the embodiments above.

0041<figref idref="DRAWINGS">FIG. 4</figref> illustrates an example of such an alternative embodiment. In <figref idref="DRAWINGS">FIG. 4</figref>, a zener diode <b>122</b> can be used instead of the shunt regulator <b>118</b> as the over-voltage protection controller <b>114</b>. The zener diode <b>122</b> is advantageously chosen such that its threshold voltage is equal to or slightly less than the predetermined voltage at or over which over-voltage will occur. In this embodiment, it is preferable to have a switch <b>124</b> disposed between the zener diode <b>122</b> and node C. The switch <b>124</b> is closed during charging and open otherwise, so that power is dissipated in the zener diode <b>122</b> only while charging the energy storage device <b>102</b>. As one skilled in the art can appreciate, little current flows through the zener diode <b>122</b> as long as the voltage of the energy storage device <b>102</b> remains below the threshold voltage of the zener diode. If the voltage rises above the threshold voltage of the zener diode <b>122</b>, exponentially greater current flows through the zener diode. Either the zener diode itself, or a combination of the zener diode <b>122</b> and a series resistor or resistors (not shown in <figref idref="DRAWINGS">FIG. 4</figref>), dissipates this excess energy. In the case of using the zener diode <b>122</b> by itself, it is both over-voltage protection controller <b>114</b> and over-voltage protection device <b>116</b>. Where the zener diode <b>122</b> is used in conjunction with a resistor, or a plurality of resistors, (not shown in <figref idref="DRAWINGS">FIG. 4</figref>) the over-voltage protection device <b>116</b> functionally comprises both the zener diode <b>122</b> and the resistor.

0042There are, however, further alternative embodiments that may be considered in the case where the energy storage device comprises a plurality of energy storage components. Such embodiments cannot be implemented with the energy storage device comprising only one energy storage component. <figref idref="DRAWINGS">FIGS. 5 and 6</figref> illustrate such exemplary alternative embodiments. Although these figures illustrate an energy storage device comprising two energy storage components, the designs may be employed in circuits having more than two energy storage components, with appropriate modifications being apparent to one of ordinary skill in the art. For example, if energy storage components are provided in multiples of two, circuits such as those illustrated in <figref idref="DRAWINGS">FIGS. 5 and 6</figref> may be connected in parallel with each pair of energy storage components. Alternatively, the over-voltage protection device may be suitably connected to more than two energy storage components, as long as the properties of the over-voltage protection device are selected such that it can handle possible over-voltage from each of the energy storage components to which it is connected.

0043<figref idref="DRAWINGS">FIG. 5</figref> illustrates an alternative embodiment of the invention. In this figure, energy storage device <b>126</b> comprises the energy storage components <b>102</b>, <b>104</b>, which are connected to the over-voltage protection circuit <b>112</b>. In <figref idref="DRAWINGS">FIG. 5</figref>, the over-voltage protection controller <b>114</b> comprises switches <b>128</b>, <b>130</b>, and the over-voltage protection device <b>116</b> comprises capacitor <b>132</b>. These switches <b>128</b>, <b>130</b> could be, for example, FETs, relay switches, BJTs, MUXs, or any other suitable means as described earlier. The switches <b>128</b>, <b>130</b> are connected to a capacitor <b>132</b> in order to protect against over-voltage by balancing the charge between the energy storage components. When the energy storage components <b>102</b>, <b>104</b> are charged and one energy storage component is at or above the predetermined voltage, the switches <b>128</b>, <b>130</b> are actuated and connect or disconnect the capacitor <b>132</b> to each energy storage component <b>102</b>, <b>104</b> in order to balance the charge between them.

0044In this embodiment, the switches <b>128</b>, <b>130</b> are actuated in phase with one another as long as over-voltage occurs. Over-voltage detectors <b>134</b> and <b>136</b> preferably control such actuation for switches <b>128</b>, <b>130</b> respectively. In this case, the over-voltage detector performs the functions of both detecting when over-voltage occurs, and controlling the actuation of the switch. A single integral over-voltage detector can alternatively perform the functions of the two over-voltage detectors <b>134</b> and <b>136</b>.

0045An advantage of this embodiment is that any excessive charge is transferred from the energy storage component with greater charge to the energy storage component with lesser charge and such excessive charge is not dissipated as it is across the resistors in <figref idref="DRAWINGS">FIG. 1</figref>. For example, if energy storage component <b>102</b> were at or over the predetermined voltage, switches <b>128</b> and <b>130</b> would connect capacitor <b>132</b> in parallel to energy storage component <b>102</b>, so that the charge is then transferred to the capacitor <b>132</b>. Later, switches <b>128</b> and <b>130</b> would connect capacitor <b>132</b> to energy storage component <b>104</b> and charge would transfer to energy storage component <b>104</b> since its voltage is lower that that of energy storage component <b>102</b>. Once again, the actuation of the switches is preferably controlled as described above.

0046Alternatively, instead of using the over-voltage detectors <b>134</b> and <b>136</b>, the circuit can comprise an actuating means (not shown in <figref idref="DRAWINGS">FIG. 5</figref>) that actuates the connection and disconnection of the capacitor <b>132</b> to each energy storage component <b>102</b>, <b>104</b> at a regular time interval. This provides for automatic charge balancing without the need for the over-voltage detectors <b>134</b>, <b>136</b>.

0047In a case such as in <figref idref="DRAWINGS">FIG. 5</figref> where the over-voltage protection device <b>116</b> temporarily stores charge associated with drawn excess charge, a dissipation controller <b>138</b> is preferably provided as part of the over-voltage protection circuit <b>114</b>. This dissipation controller <b>138</b> enables the charge stored in the over-voltage protection device <b>116</b> to be dissipated in an appropriate manner, as will be well known to one skilled in the art. For example, the dissipation controller can comprise a circuit having a dissipation switch that connects said over-voltage protection device to ground in order to dissipate the stored voltage. Preferably, this dissipation controller will also comprise a dissipation control mechanism that actuates the dissipation switch so as to connect and disconnect the over-voltage protection device <b>116</b> from the dissipation controller according to appropriate conditions. In an alternative embodiment, a single integral controller may perform all the functions of over-voltage detectors <b>134</b>, <b>136</b> as well as those of the dissipation controller <b>138</b>.

0048<figref idref="DRAWINGS">FIG. 6</figref> is another alternative embodiment of the invention. This figure illustrates a circuit that operates similarly to the circuit in <figref idref="DRAWINGS">FIG. 5</figref>, but has an improved efficiency over the embodiment in <figref idref="DRAWINGS">FIG. 5</figref>. In <figref idref="DRAWINGS">FIG. 6</figref>, energy storage device <b>126</b> comprises the energy storage components <b>102</b>, <b>104</b>, which are connected to the over-voltage protection circuit <b>112</b>. Each energy storage component <b>102</b>, <b>104</b> is connected to switch <b>128</b>, <b>130</b>. The switches <b>128</b>, <b>130</b> alternatively connect the respective energy storage component to an inductor <b>140</b>, thus moving any excess charge between the energy storage components. In this embodiment, the switches <b>128</b>, <b>130</b> are actuated out of phase with one another and cannot both be closed at the same time.

0049Diodes <b>142</b> and <b>144</b> conduct during the brief interval when one switch has opened and the other has not yet closed while there is energy stored in inductor <b>140</b>. When an energy storage component charges to (or just over) the predetermined voltage, the factor affecting which switch will close first is which energy storage component has the greater voltage.

0050For example, consider the situation where, within the circuit in <figref idref="DRAWINGS">FIG. 6</figref>, energy storage component <b>104</b> is at or just above the predetermined voltage and has a greater voltage than energy storage component <b>102</b>. Then, switch <b>130</b> closes for a period of time to energize, but not saturate, inductor <b>140</b>. Later, switch <b>130</b> opens and diode <b>142</b> immediately begins to conduct, because there is energy stored in inductor <b>140</b>. Switch <b>128</b> subsequently closes, short-circuiting diode <b>142</b> to improve efficiency, since switch <b>128</b> has a lower voltage across it than diode <b>142</b> when it is closed, and thereby transferring charge to energy storage component <b>102</b>.

0051In <figref idref="DRAWINGS">FIG. 6</figref>, since the over-voltage protection device <b>116</b> temporarily stores charge associated with drawn excess charge, a dissipation controller <b>138</b> is preferably provided as part of the over-voltage protection circuit <b>114</b>. This dissipation controller <b>138</b> enables the charge stored in the over-voltage protection device <b>116</b> to be dissipated in an appropriate manner, as will be well known to one skilled in the art. For example, the dissipation controller can comprise a circuit having a dissipation switch that connects said over-voltage protection device to ground in order to dissipate the stored voltage. Preferably, this dissipation controller will also comprise a dissipation control mechanism that actuates the dissipation switch so as to connect and disconnect the over-voltage protection device <b>116</b> from the dissipation controller according to appropriate conditions. In an alternative embodiment, a single integral controller may perform all the functions of over-voltage detectors <b>134</b>, <b>136</b> as well as those of the dissipation controller <b>138</b>.

0052The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.